1. Field of the Invention
The present invention concerns a metallic gasket disposed between joining faces of a cylinder head and a cylinder block in an internal combustion engine to prevent combustion gas blow through and cooling water and lubricant or the like from leaking.
2. Description of the Prior Art
In an internal combustion engine, a gasket is interposed between a cylinder block and a cylinder head to provide a seal therebetween. In particular, a seal at the periphery of a combustion chamber, that is, a cylinder bore, is important and, if the seal is defective at that portion, the combustion gas in the bore blows out to decrease the gas pressure efficiency and the expected power output can not be obtained.
The gasket materials for a gasoline engine use a mixture of asbestos and rubber having a spiked steel plate embedded therein. A bore side peripheral edge covered with a stainless steel plate and the peripheral edge of bolt holes covered with a copper plate, respectively, are used as a countermeasure for a gap between the edge seal and the joining faces.
Further, for a diesel engine, a material comprising a mixture of asbestos and rubber and a wire gauze embedded therein is used as a countermeasure for a gap at the edge seal and the joining faces as described above and a fire ring is further engaged along the edge of the bore.
Such gaskets are referred to as soft gaskets. However, since asbestos is used in these gaskets, its further use has become restricted, particularly, in view of environmental problems.
As far as the inventor knows, synthetic fibers are used instead of asbestos in gasoline engines. Further, for diesel engines, a structure comprising a steel laminate including a steel plate formed with a minute bead, a stainless steel subplate stacked thereon and a steel wire core is disposed to the bore peripheral edge. The subplate is folded back to cover the wire on the inside and extends along the upper surface of the edge of the bore hole. A baked rubber seal is disposed at the bolt holes.
Further, there is also a prior art gasket as shown in FIG. 8 in which a stopper 4 comprising a shim is formed at a flat portion adjacent a bead 2 of an elastic steel or iron base plate 1 on the side of a bore hole, a gasket as shown in FIG. 7 in which a stopper 3 is formed by arc plasma spray at a flat portion adjacent a bead 2 of an elastic base plate 1 on the side of a bore hole, and a gasket with two intermediate plates bent outside with each other to open at a bore edge on which a stopper is also formed by arc plasma spray, not illustrated.
The soft gaskets are not satisfactory for providing reduced thickness or endurance to cope with decreasing distance between bores required by the recent trend of engine weight and size reduction, material recycling, heat resistance, high performance, cost reduction and freedom of design. Eventually, the demand for metallic gaskets will increase in the automobile industry.
By the way, in a metallic gasket, a bead is formed along the periphery of holes, in particular, at the periphery of cylinder bore holes. When a metallic gasket is tightened between a cylinder head and a cylinder block, the bead is elastically deformed to function as a seal. Further, a stopper is formed in an annular shape from the bead toward the cylinder bore hole, that is, at the periphery of the bore hole in the metallic gasket. The stopper, by its thickness, suppresses vibrations at the joined area between the cylinder block and the cylinder head caused by the engine operation so that it also functions as a sealing member to prevent fatigue failure due to engine vibration. In the metallic gasket, a complete seal is provided by obtaining a primary seal with the stopper and a secondary bead seal compensated with the durability by the stopper. The stopper must have a thickness corresponding to a gap in the engine, that is, a gap between the joined faces of the cylinder block and the cylinder head.
Recently, with performance improvement due to the progress in the engine technology, fuel economy and size and weight reduction, the distance between bores has been reduced and the rigidity of the engine has been reduced considerably. As a result, it is necessary to further increase the tightening pressure in view of the requirement for sealing an increasing explosive pressure and, inevitably, the spring force of the bead has to be increased. When such a highly elastic gasket is placed between the joining faces of the cylinder block and the cylinder and tightened with an enormous force, the surface pressure increases excessively near the bolts due to the tightening and the reduced gap between the joining faces. On the other hand, the surface pressure decreases near the mid portion between the bolts and tends to increase the gap between the joining faces.
Accordingly, when a gasket is fabricated with a stopper by using a shim having an identical thickness over the entire surface, the surface pressure between the joining faces at the periphery of the cylinder bore can not be uniform and, if tightening is applied excessively to attain a seal over the entire periphery, distortions occur at the periphery of the cylinder bore which deform the cylinder bore. Then it becomes difficult to ensure circularity of the cylinder bore due to uneven tightening pressure.
A stopper formed by arc plasma spray can cope with the difference in thickness depending on its position; but it has a drawback in that since the welding efficiency of the plasma spray is extremely low, the dimensional accuracy can not be easily ensured and, further, sufficient sealing can not be obtained with a low spray density.
Although examples of internal combustion engines have been described, that is, a gasoline engine or a diesel engine, similar problems, of course, exist due to unevenness of the surface pressure at the joining faces of a crank case or a differential case.
As an example of the metallic gasket in the relevant prior art, the inventor has accomplished inventions disclosed in Japanese Patent Laid-Open Hei 1 (1989)-104953, European Patent Application Laid-Open No. 0306766 and Japanese Patent Publication Hei 2 (1990) 58502 and, further, is aware of Japanese Patent Laid-Open Sho 62 (1987) -155376 and Japanese Patent Publications Hei 3 (1991) -20626 and Hei 4 (1993) -40540.